To solve these problems, German Offenlegungsschrift No. 26 31 622.3 has proposed that the liquid be fed into a treatment machine in the form of a tubular envelope, the solids being introduced into the center thereof. This tubular liquid envelope which runs down, in the form of a liquid film, on the inner wall of the feed branch surrounds the solids stream, which is to be introduced into the treatment machine, and, due to its direction of flow, prevents the solids from penetrating this envelope transversely to its direction of flow. This solution according to the German Application has proved extremely successful in practice.